ALPHA-2 Adrenergic receptors have with (-)-3H-epinephrine (EPI), 3H-p-aminoclonidine (PAC) and 3H-yohimbine (YOH), in human platelet membranes, and, with the first two radioligands, in membranes from cultured neuroblastoma X glioma (NG 108-15) hybrid clonal cells. Receptors in these tissues are coupled to adenylate cyclase in an inhibitory manner. 3H-Pac and 3H-EPI binding sites had very high (nM) affinity for agonists, and binding of these ligands was supressed by guanine nucleotides. At 3H-YOH sites, agonists had lower affinity and exhibited apparent negative cooperativity. Antagonist affinities at the three binding sites were similar to each other, and to potencies in reversing agonist effects on adenylate cyclase. These binding data at present suggest that the alpha 2-receptor in these tissues and in brain exixts in at least two conformational states distinguished by affinities of agonists. 3H-Yoh labels the entire population of alpha2-receptors, wheras 3H-PAC and 3H-EPI selectively label a high-affinity component. Brain alpha2-receptor regulation was examined after NE dorsal bundle lesion, or after in vitro or in vivo treatments which cause central Beta-receptor subsensitivity. Cortical alpha2-receptor binding was increased after DB lesion, suggesting a postsynaptic location. Beta-Receptor subsensitivity produced by incubation of cortical slices with isoproterenol, chronic antidepressant treatment, or immobilization stress, was accompanied by increases in the number of 3H-PAC or 3H-clonidine sites. This increase may reflect a true alpha2-receptor supersensitivity, or a shift in equilibrium between the different conformational states of the alpha2-receptor.